On the Origin of Species by Means of Natural Selection : Chapter XI-C

In illustrating what, as I believe, actually took place during the Glacial
period, I assumed that at its commencement the arctic productions were as
uniform round the polar regions as they are at the present day. But the
foregoing remarks on distribution apply not only to strictly arctic forms,
but also to many sub-arctic and to some few northern temperate forms, for
some of these are the same on the lower mountains and on the plains of
North America and Europe; and it may be reasonably asked how I account for
the necessary degree of uniformity of the sub-arctic and northern temperate
forms round the world, at the commencement of the Glacial period. At the
present day, the sub-arctic and northern temperate productions of the Old
and New Worlds are separated from each other by the Atlantic Ocean and by
the extreme northern part of the Pacific. During the Glacial period, when
the inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We have
good reason to believe that during the newer Pliocene period, before the
Glacial epoch, and whilst the majority of the inhabitants of the world were
specifically the same as now, the climate was warmer than at the present
day. Hence we may suppose that the organisms now living under the climate
of latitude 60 deg, during the Pliocene period lived further north under
the Polar Circle, in latitude 66 deg-67 deg; and that the strictly arctic
productions then lived on the broken land still nearer to the pole. Now if
we look at a globe, we shall see that under the Polar Circle there is
almost continuous land from western Europe, through Siberia, to eastern
America. And to this continuity of the circumpolar land, and to the
consequent freedom for intermigration under a more favourable climate, I
attribute the necessary amount of uniformity in the sub-arctic and northern
temperate productions of the Old and New Worlds, at a period anterior to
the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to large,
but partial oscillations of level, I am strongly inclined to extend the
above view, and to infer that during some earlier and still warmer period,
such as the older Pliocene period, a large number of the same plants and
animals inhabited the almost continuous circumpolar land; and that these
plants and animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the commencement of
the Glacial period. We now see, as I believe, their descendants, mostly in
a modified condition, in the central parts of Europe and the United States.
On this view we can understand the relationship, with very little identity,
between the productions of North America and Europe,--a relationship which
is most remarkable, considering the distance of the two areas, and their
separation by the Atlantic Ocean. We can further understand the singular
fact remarked on by several observers, that the productions of Europe and
America during the later tertiary stages were more closely related to each
other than they are at the present time; for during these warmer periods
the northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered impassable
by cold, for the inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they must have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and have had to compete with them;
and in the other great region, with those of the Old World. Consequently
we have here everything favourable for much modification,--for far more
modification than with the Alpine productions, left isolated, within a much
more recent period, on the several mountain-ranges and on the arctic lands
of the two Worlds. Hence it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical races,
and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as specifically
distinct.

As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in areas completely sundered. Thus, I think, we can understand the
presence of many existing and tertiary representative forms on the eastern
and western shores of temperate North America; and the still more striking
case of many closely allied crustaceans (as described in Dana's admirable
work), of some fish and other marine animals, in the Mediterranean and in
the seas of Japan,--areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the theory
of creation. We cannot say that they have been created alike, in
correspondence with the nearly similar physical conditions of the areas;
for if we compare, for instance, certain parts of South America with the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their inhabitants
utterly dissimilar.

But we must return to our more immediate subject, the Glacial period. I am
convinced that Forbes's view may be largely extended. In Europe we have
the plainest evidence of the cold period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may infer,
from the frozen mammals and nature of the mountain vegetation, that Siberia
was similarly affected. Along the Himalaya, at points 900 miles apart,
glaciers have left the marks of their former low descent; and in Sikkim,
Dr. Hooker saw maize growing on gigantic ancient moraines. South of the
equator, we have some direct evidence of former glacial action in New
Zealand; and the same plants, found on widely separated mountains in this
island, tell the same story. If one account which has been published can
be trusted, we have direct evidence of glacial action in the south-eastern
corner of Australia.

Looking to America; in the northern half, ice-borne fragments of rock have
been observed on the eastern side as far south as lat. 36 deg-37 deg, and
on the shores of the Pacific, where the climate is now so different, as far
south as lat. 46 deg; erratic boulders have, also, been noticed on the
Rocky Mountains. In the Cordillera of Equatorial South America, glaciers
once extended far below their present level. In central Chile I was
astonished at the structure of a vast mound of detritus, about 800 feet in
height, crossing a valley of the Andes; and this I now feel convinced was a
gigantic moraine, left far below any existing glacier. Further south on
both sides of the continent, from lat. 41 deg to the southernmost
extremity, we have the clearest evidence of former glacial action, in huge
boulders transported far from their parent source.

We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have
good evidence in almost every case, that the epoch was included within the
latest geological period. We have, also, excellent evidence, that it
endured for an enormous time, as measured by years, at each point. The
cold may have come on, or have ceased, earlier at one point of the globe
than at another, but seeing that it endured for long at each, and that it
was contemporaneous in a geological sense, it seems to me probable that it
was, during a part at least of the period, actually simultaneous throughout
the world. Without some distinct evidence to the contrary, we may at least
admit as probable that the glacial action was simultaneous on the eastern
and western sides of North America, in the Cordillera under the equator and
under the warmer temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it is difficult to avoid
believing that the temperature of the whole world was at this period
simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of
longitude.

On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty and fifty of the flowering
plants of Tierra del Fuego, forming no inconsiderable part of its scanty
flora, are common to Europe, enormously remote as these two points are; and
there are many closely allied species. On the lofty mountains of
equatorial America a host of peculiar species belonging to European genera
occur. On the highest mountains of Brazil, some few European genera were
found by Gardner, which do not exist in the wide intervening hot countries.
So on the Silla of Caraccas the illustrious Humboldt long ago found species
belonging to genera characteristic of the Cordillera. On the mountains of
Abyssinia, several European forms and some few representatives of the
peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a
very few European species, believed not to have been introduced by man, and
on the mountains, some few representative European forms are found, which
have not been discovered in the intertropical parts of Africa. On the
Himalaya, and on the isolated mountain-ranges of the peninsula of India, on
the heights of Ceylon, and on the volcanic cones of Java, many plants
occur, either identically the same or representing each other, and at the
same time representing plants of Europe, not found in the intervening hot
lowlands. A list of the genera collected on the loftier peaks of Java
raises a picture of a collection made on a hill in Europe! Still more
striking is the fact that southern Australian forms are clearly represented
by plants growing on the summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend along the heights of
the peninsula of Malacca, and are thinly scattered, on the one hand over
India and on the other as far north as Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
of European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable 'Introduction to the Flora of New Zealand,' by
Dr. Hooker, analogous and striking facts are given in regard to the plants
of that large island. Hence we see that throughout the world, the plants
growing on the more lofty mountains, and on the temperate lowlands of the
northern and southern hemispheres, are sometimes identically the same; but
they are much oftener specifically distinct, though related to each other
in a most remarkable manner.

This brief abstract applies to plants alone: some strictly analogous facts
could be given on the distribution of terrestrial animals. In marine
productions, similar cases occur; as an example, I may quote a remark by
the highest authority, Prof. Dana, that 'it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world.' Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of
the intertropical regions, are not arctic, but belong to the northern
temperate zones. As Mr. H. C. Watson has recently remarked, 'In receding
from polar towards equatorial latitudes, the Alpine or mountain floras
really become less and less arctic.' Many of the forms living on the
mountains of the warmer regions of the earth and in the southern hemisphere
are of doubtful value, being ranked by some naturalists as specifically
distinct, by others as varieties; but some are certainly identical, and
many, though closely related to northern forms, must be ranked as distinct
species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that the
whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. The Glacial period, as
measured by years, must have been very long; and when we remember over what
vast spaces some naturalised plants and animals have spread within a few
centuries, this period will have been ample for any amount of migration.
As the cold came slowly on, all the tropical plants and other productions
will have retreated from both sides towards the equator, followed in the
rear by the temperate productions, and these by the arctic; but with the
latter we are not now concerned. The tropical plants probably suffered
much extinction; how much no one can say; perhaps formerly the tropics
supported as many species as we see at the present day crowded together at
the Cape of Good Hope, and in parts of temperate Australia. As we know
that many tropical plants and animals can withstand a considerable amount
of cold, many might have escaped extermination during a moderate fall of
temperature, more especially by escaping into the warmest spots. But the
great fact to bear in mind is, that all tropical productions will have
suffered to a certain extent. On the other hand, the temperate
productions, after migrating nearer to the equator, though they will have
been placed under somewhat new conditions, will have suffered less. And it
is certain that many temperate plants, if protected from the inroads of
competitors, can withstand a much warmer climate than their own. Hence, it
seems to me possible, bearing in mind that the tropical productions were in
a suffering state and could not have presented a firm front against
intruders, that a certain number of the more vigorous and dominant
temperate forms might have penetrated the native ranks and have reached or
even crossed the equator. The invasion would, of course, have been greatly
favoured by high land, and perhaps by a dry climate; for Dr. Falconer
informs me that it is the damp with the heat of the tropics which is so
destructive to perennial plants from a temperate climate. On the other
hand, the most humid and hottest districts will have afforded an asylum to
the tropical natives. The mountain-ranges north-west of the Himalaya, and
the long line of the Cordillera, seem to have afforded two great lines of
invasion: and it is a striking fact, lately communicated to me by Dr.
Hooker, that all the flowering plants, about forty-six in number, common to
Tierra del Fuego and to Europe still exist in North America, which must
have lain on the line of march. But I do not doubt that some temperate
productions entered and crossed even the lowlands of the tropics at the
period when the cold was most intense,--when arctic forms had migrated some
twenty-five degrees of latitude from their native country and covered the
land at the foot of the Pyrenees. At this period of extreme cold, I
believe that the climate under the equator at the level of the sea was
about the same with that now felt there at the height of six or seven
thousand feet. During this the coldest period, I suppose that large spaces
of the tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base of
the Himalaya, as graphically described by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator,
would re-migrate northward or southward towards their former homes; but the
forms, chiefly northern, which had crossed the equator, would travel still
further from their homes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe from geological evidence
that the whole body of arctic shells underwent scarcely any modification
during their long southern migration and re-migration northward, the case
may have been wholly different with those intruding forms which settled
themselves on the intertropical mountains, and in the southern hemisphere.
These being surrounded by strangers will have had to compete with many new
forms of life; and it is probable that selected modifications in their
structure, habits, and constitutions will have profited them. Thus many of
these wanderers, though still plainly related by inheritance to their
brethren of the northern or southern hemispheres, now exist in their new
homes as well-marked varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical plants and allied forms have apparently migrated from the north
to the south, than in a reversed direction. We see, however, a few
southern vegetable forms on the mountains of Borneo and Abyssinia. I
suspect that this preponderant migration from north to south is due to the
greater extent of land in the north, and to the northern forms having
existed in their own homes in greater numbers, and having consequently been
advanced through natural selection and competition to a higher stage of
perfection or dominating power, than the southern forms. And thus, when
they became commingled during the Glacial period, the northern forms were
enabled to beat the less powerful southern forms. Just in the same manner
as we see at the present day, that very many European productions cover the
ground in La Plata, and in a lesser degree in Australia, and have to a
certain extent beaten the natives; whereas extremely few southern forms
have become naturalised in any part of Europe, though hides, wool, and
other objects likely to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata, and during the last
thirty or forty years from Australia. Something of the same kind must have
occurred on the intertropical mountains: no doubt before the Glacial
period they were stocked with endemic Alpine forms; but these have almost
everywhere largely yielded to the more dominant forms, generated in the
larger areas and more efficient workshops of the north. In many islands
the native productions are nearly equalled or even outnumbered by the
naturalised; and if the natives have not been actually exterminated, their
numbers have been greatly reduced, and this is the first stage towards
extinction. A mountain is an island on the land; and the intertropical
mountains before the Glacial period must have been completely isolated; and
I believe that the productions of these islands on the land yielded to
those produced within the larger areas of the north, just in the same way
as the productions of real islands have everywhere lately yielded to
continental forms, naturalised by man's agency.

I am far from supposing that all difficulties are removed on the view here
given in regard to the range and affinities of the allied species which
live in the northern and southern temperate zones and on the mountains of
the intertropical regions. Very many difficulties remain to be solved. I
do not pretend to indicate the exact lines and means of migration, or the
reason why certain species and not others have migrated; why certain
species have been modified and have given rise to new groups of forms, and
others have remained unaltered. We cannot hope to explain such facts,
until we can say why one species and not another becomes naturalised by
man's agency in a foreign land; why one ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own homes.

I have said that many difficulties remain to be solved: some of the most
remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed.
I will only say that as far as regards the occurrence of identical species
at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia,
I believe that towards the close of the Glacial period, icebergs, as
suggested by Lyell, have been largely concerned in their dispersal. But
the existence of several quite distinct species, belonging to genera
exclusively confined to the south, at these and other distant points of the
southern hemisphere, is, on my theory of descent with modification, a far
more remarkable case of difficulty. For some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the Glacial period for their migration, and for their
subsequent modification to the necessary degree. The facts seem to me to
indicate that peculiar and very distinct species have migrated in radiating
lines from some common centre; and I am inclined to look in the southern,
as in the northern hemisphere, to a former and warmer period, before the
commencement of the Glacial period, when the antarctic lands, now covered
with ice, supported a highly peculiar and isolated flora. I suspect that
before this flora was exterminated by the Glacial epoch, a few forms were
widely dispersed to various points of the southern hemisphere by occasional
means of transport, and by the aid, as halting-places, of existing and now
sunken islands, and perhaps at the commencement of the Glacial period, by
icebergs. By these means, as I believe, the southern shores of America,
Australia, New Zealand have become slightly tinted by the same peculiar
forms of vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate on
geographical distribution. I believe that the world has recently felt one
of his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the present
distribution both of the same and of allied forms of life can be explained.
The living waters may be said to have flowed during one short period from
the north and from the south, and to have crossed at the equator; but to
have flowed with greater force from the north so as to have freely
inundated the south. As the tide leaves its drift in horizontal lines,
though rising higher on the shores where the tide rises highest, so have
the living waters left their living drift on our mountain-summits, in a
line gently rising from the arctic lowlands to a great height under the
equator. The various beings thus left stranded may be compared with savage
races of man, driven up and surviving in the mountain-fastnesses of almost
every land, which serve as a record, full of interest to us, of the former
inhabitants of the surrounding lowlands.